1. Field of the Invention
The invention relates to implantable devices or endoluminal prostheses, such as stents, and methods of forming a sheath on such devices.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially press against the atherosclerotic plaque of the lesion for remodeling of the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient""s vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings, which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may necessitate another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable, intraluminal prosthesis, one example of which is a stent, is implanted in the lumen to maintain the vascular patency.
Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small cavities via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in the patent literature disclosing stents that have been applied in PTCA procedures include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis is still a significant clinical problem with rates ranging from 20-40%. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or even toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
One method of medicating a stent involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer.
Depending on the physiological mechanism targeted, the therapeutic substance may be required to be released at an efficacious concentration for an extended duration of time. Increasing the quantity of the therapeutic substance in the polymeric coating can lead to poor coating mechanical properties, inadequate coating adhesion, and overly rapid rate of release. Increasing the quantity of the polymeric compound by producing a thicker coating can perturb the geometrical and mechanical functionality of the stent, as well as limit the procedure for which the stent can be used.
It is desirable to increase the residence time of a substance at the site of implantation, at a therapeutically useful concentration, without the application of a thicker coating. It is also desirable to be able to increase the quantity of the therapeutic substance carried by the polymeric layer without perturbing the mechanical properties of the coating, such as adhesion of the polymer to the stent substrate.
The present invention provides a method of forming a sheath on a prosthesis, e.g., a stent. The method includes providing a prosthesis for implantation in a biological passageway. The prosthesis has a longitudinally extending central bore for allowing a fluid to travel through the prosthesis in the passageway and includes a plurality of interconnected struts separated by gaps. The method includes forming a sheath circumscribing at least a portion of the prosthesis. The sheath covers the gaps underlying the sheath. In one embodiment, the sheath contains an active ingredient. In other embodiments, the sheath contains radiopaque elements, radioactive isotopes, nucleic acids, or proteins. The method can further include removing a portion of the sheath positioned over some of the gaps to form a pattern of interstices dispersed between the struts for allowing a fluid that flows through the central bore to seep through the sheath.
Also provided is a stent. The stent includes a generally tubular structure having a plurality of interconnected struts. A sheath is disposed about an outer surface of the generally tubular structure. In addition, the stent includes a pattern of interstices disposed in the sheath, interspersed between the struts. The interstices allow a fluid to flow through the sheath.
Also provided is a method for increasing the amount of a polymeric coating, without increasing the thickness of the coating, for a stent having struts separated by gaps. The method includes applying a composition including a polymeric compound and a solvent to the stent. The method also includes removing the solvent from the composition applied to the stent to form a coating. The coating covers the struts and the gaps between the struts so as to increase the quantity of the polymeric material supported by the stent without increasing the thickness of the coating on the stent. The method can also include removing a portion of the coating deposited over at least one of the gaps to create an opening in the coating. The size of the opening is smaller than the size of the gap.